Increases in the size of boilers has led to a tendency for the electrostatic precipitators used for the purification of their exhaust gases to determine the maximum width of the installation. Increased throughput cannot be dealt with by increasing only the height of the precipitators because elongation of the collector electrodes beyond a length of about 15 meters is connected with disadvantages on electrical grounds, on grounds of flow technique, and of difficulties of erection. Apart from the disadvantages of requirement for width there arise in the case of wide filter installation problems of distribution of gas flow and of high capital cost.
A multistage construction, with the gas flowing through horizontally, serves, in principle, the purpose of diminishing the floor space requirement of the installation and of enabling more uniform distribution of the gas over the whole cross-section of the flow.
However, in known proposals for such multistage electrostatic precipitators [e.g. in accordance with DTOS (German Laid Open Specification) 1 457 177], precipitator bays in the upper tiers of the electrostatic precipitator installation are equipped with their own dust bunkers which take up a considerable part of the overall height of the structure.
In U.S. Pat. No. 2,588,364 (De Giorgi) there was a solution of the problem of vertical compactness when one tier of electrodes was arranged one above the other, by using ducts arranged between electrodes of the lower tier as means for conducting dust precipitated from the upper tier to a hopper at the base of the construction. However, De Giorgi saw this as a single electrostatic precipitator arrangement, i.e. a multi-field single precipitator. He failed to consider the electrodes of different tiers as if they were separate precipitators and thereby failed to achieve even the basis of the present invention. In column 9, second paragraph of this U.S. patent specification it is stated that the discharge electrodes are all connected with a common rectifier. This patent specification thus does not disclose high voltage supplies, which can be regulated separately, for separate stages which are traversed by partial gas volumes. De Giorgi also shows successive precipitator fields in series. A similar disclosure is seen in U.S. Pat. No. 2,626,676 of Phyl et al.
For a given temperature, flow rate and dust content of a gas, the efficiency of a precipitator field is directly proportional to the potential in the electrodes. Efficiency is considered as the statistical probability that a given dust particle will have been discharged from the gas before it leaves the field. However, voltage cannot be increased indefinitely because too high a voltage will cause breakdowns in insulation and hence sparkovers which are unacceptable because of damage caused to electrodes. There would also be the danger that greatly increased voltage will cause the output (wattage) of the generating set to be exceeded at a given amperage i.e. at a given ionization current taken by the dust being charged or discharged.
Voltage control is therefore applied by automatic devices, well known in the art, (see, for example, U.S. Pat. No. 3,166,705) which continually sense the current flowing in each field so as to detect the number and intensity of sparkovers in the time unit and which, when excessive storage of these measured values is detected, are self-regulating so as to lower the voltage unit it falls. Once the reduced level has been reached, such a device attempts to build up voltage again; in effect the control devices build up to a maximum voltage in combination with a predetermined low number and intensity of sparkovers in the time unit, resulting in the most favorable performance.
Also, gas flow is seldom exactly uniform. It is normally stratified into layers of different temperature or dust load. Therefore it is efficient to have each field as small as possible and with individual control, so as to respond to such inequalities.
Another aspect of efficiency concerns rapping. Rapping is most efficient if it is done when there is a certain thickness of layer of dust accumulated on the collector electrodes. Also one tries to keep the rate of rapping as low as possible because each rap will necessarily liberate a certain amount of dust back into the gas stream; so it is inefficient to rap more often than is needed. De Giorgi suggests therefore to set rapping apparatus to operate at rates which are different as between different serial stages and which agree with expected rate of accumulation of dust in each respective stage.
However, De Giorgi disclosed only separate rapping devices for the stages which are arranged in series, while the stages which are arranged above one another are contrary to the object of the present invention, rapped jointly.